1. Field of the Invention
This invention relates generally to an overrunning, one-way coupling, such as a brake or clutch, whose engaged and disengaged states are selectively controllable.
2. Description of the Prior Art
Automatic transmissions employ hydraulically-actuated clutches and brakes to control power flow and establish the operating gear. A one-way clutch carries torque in one direction while overrunning in the opposite direction. A selectable one-way clutch can overrun in both directions, and can be commanded to lock clockwise, counterclockwise, or both directions. Electromagnetic clutches use electromagnetism to actuate the locking element or strut, as opposed to hydraulic pressure, a mechanical linkage, or centrifugal force.
One-way clutches typically include two races and a locking element, sometimes called a strut or rocker. The locking element or strut is a movable component that will wedge between the races and transfer torque in one direction. When torque is reversed, the locking element will “tuck” or rotate out of the locked position. One race (the pocket plate) houses the locking element and can be either rotating or stationary. The other race (the cam plate) contains features to interact with the locking element and transfer torque. This race also can be either rotating or stationary.
In an electromagnetic one-way clutch, the locking element is controlled through the use of electromagnetism. When current is applied to a coiled conductor an electromagnet is energized to either engage or disengage the locking element.
In order to function properly electromagnetic one-way clutches require a gap between the locking element and the magnet poles to generate a force necessary to actuate the locking element or rocker When the coil is energized, the rocker must rotate a particular distance to change the engagement state. This rotation closes the gap between the rocker and the poles of the coil. Consequently the gap between the poles and the rocker is greatest prior to energizing. To minimize the size of the coil it is beneficial to manage this gap.
Magnetically non-permeable material is frequently used to prevent flux leakage, but if too much flux leaks through the race rather than jumping the gap to the rocker the magnitude of the force generated may be insufficient to displace the rocker.
It is desirable to place the coil of an electromagnetic clutch as close as possible to the locking element. However doing so puts the coil in the area of the race that carries most force and deflection, thereby risking damage to the coil. Moving the coil to a safer location however compromises flux generation.